"Our results have important public health and clinical implications," the researchers write.

Folic acid is found naturally in liver and raw vegetables and plays an important role in fetal growth. In the past, both folate deficiency and folic acid supplements have been reported to increase the risk of spontaneous abortion (miscarriage) before the 20th week of pregnancy. But these results have been inconclusive.

The researchers conducted their own study to determine whether plasma folate levels affected the chance of a spontaneous abortion.

The study included 468 women who had a spontaneous abortion and 921 controls. The researchers took plasma folate measurements between six to 12 gestational weeks.

The study was conducted over three years between January 1996 and December 1998. The participants all lived in Sweden where food is not fortified by folic acid.

Women with a low folate level  equal to, or less than, 2.19 nanograms/mL  were at nearly a 50 per cent increased risk of having a spontaneous abortion compared to women with plasma folate levels between 2.20 and 3.95 ng/mL.

Those women with higher folate levels - between 3.96ng/mL and higher  showed no increased risk of spontaneous abortion.

The authors were unable to identify the mechanism by which low folate levels could cause spontaneous abortion.

They conclude: "Countries that have fortified their food supplies with folic acid and those considering doing so can be reassured that fortification most likely will not increase spontaneous abortion rates and might even decrease them."

Folate also protects against cancer

An Australian study has found folate also reduces the risk of DNA damage, the underlying cause of cancer.